A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In the lithographic apparatus, use is made of a movable support to hold and position an exchangeable object such as the substrate or the patterning device. In a scanning type lithographic apparatus, a movable support is used to support the substrate in order to make the scanning movement. The patterning device may also be supported on a movable support. The movable support is able to position the substrate or patterning device with high accuracy.
To obtain a high accuracy, a known movable support is assembled from a long stroke part, movable with respect to a reference object such as a frame or balance mass, and a short stroke part, movably arranged with respect to the long stroke part. The short stroke part is configured to support the exchangeable object. The maximum stroke of the long stroke part with respect to reference object is relatively large, while the stroke of the short stroke part with respect to the long stroke part is relatively small.
A long stroke actuator is provided to actuate the long stroke part with respect to the reference object. A short stroke actuator is provided to actuate the short stroke part with respect to the long stroke part. Such long stroke actuator is for instance a linear motor, and may not be very accurate. The main task of the long stroke actuator is to bring a desired position of the exchangeable object within the reach of the short stroke actuator. The short stroke actuator is designed to position the short stroke part with high accuracy.
In order to control the position of the exchangeable object, the position of the second support system is determined by a position measurement system, for instance an interferometer system or an encoder system. This measurement is for instance performed in three planar degrees of freedom or in six degrees of freedom. The measured position is compared with a desired position. The position error, i.e. the difference between measured and desired position is fed into a controller which on the basis of this signal provides a control signal which is used to actuate the short stroke actuator.
The long stroke actuator is controlled by using a signal based on the difference between the actual position of the short stroke part and the long stroke part as an input signal for the long stroke actuator controller. The output of this controller makes the long stroke part follow the movements of the short stroke part, therewith keeping the desired position of the short stroke part within the range of the short stroke actuator.
The short stroke actuator may be of the Lorentz type to enable isolation from long-stroke vibrations. Such Lorentz type actuator has a small stiffness. Any other type of actuator having a small stiffness and high accuracy may also be used to accurately control the position of the exchangeable object support by the movable support. The input of a Lorentz actuator is an electrical current, substantially proportional to the desired force. Generally, the movable support position response to an input force is somewhat delayed because the force is integrated twice before it turns into a position. This effect, together with higher-order dynamics, limits the bandwidth of the short-stroke control loop. This limited bandwidth has a negative effect on the accuracy/settling time in the positioning of the exchangeable object held on the movable support.
The force-type-actuator in the stages may also limit the achievable feed-forward effect from one stage to the other (e.g. substrate table error fed to patterning device support). In this feed-forward, the position error of one stage needs to be differentiated twice to generate a feed-forward force, which costs one sample delay. This leads to a delayed response of the other stage, limiting positioning accuracy of the stages relative to each other.